This invention concerns the utilization of the heating values of carbonaceous fuels for the production of useful thermal, mechanical or electrical energy.
Burning coal to generate steam is one of the oldest of the industrial arts. Numerous inventions have been applied to improving its efficiency and alleviating the co-production of noxious smoke, which tends to contain unburned fuel, finely powdered ash and oxides of sulfur and nitrogen. Nevertheless, even with the latest technology, coal is considered a dirty fuel, capable only with great difficulty and expense of complying with increasingly stringent air pollution standards.
The high cost of removing sulfur oxides from conventional flue gasses has resulted in a spread between the prices of low and high sulfur coals. Moreover, the former are found, for the most part, in western states remote from the areas of greatest energy need. Thus, the market price structure provides economic incentive for the commercialization of a process able to produce steam and power from high sulfur coals without polluting the atmosphere.
Combustion of coal in conventional ways creates temperatures well above 2000.degree. F. Conventional apparatus must therefore be constructed of expensive materials capable of withstanding such temperatures. Moreover, components of the ash frequently melt (slag) forming deposits which foul parts of the apparatus, causing loss of efficiency, downtime and increased maintenance expense. A further undesirable consequence of the usual combustion temperatures is the inadvertant formation of nitrogen oxides which cannot be effectively and economically removed from flue gas with available technology.
Generation of high pressure steam does not inherently require such high temperatures since the boiling point of water at 2000 pounds per square inch is only about 635.degree. F. and at 3000 pounds per square inch about 695.degree. F.
Some experimental combustion systems, particularly those employing fluidized beds of finely divided solids at elevated pressure, permit combustion in a lower temperature range, typically 1500.degree. to 1700.degree. F. Although nitrogen oxides are thus largely avoided, expensive temperature-resistant construction is still required and new difficulties, associated with the maintenance of fluidized solids properties, erosion and removal of dust from gas streams, are entailed.
It has also been proposed to burn coal without air pollution by the indirect means of first converting it to liquid or gaseous fuel which can be desulfurized before combustion to a clean flue gas. These techniques also employ high temperature and generally share serious economic and operational drawbacks associated with coal's tendency to cake and stick when heated, the formation of tarry residues and difficulties with erosion and dust control. These techniques are further burdened by low overall thermal efficiencies.
It has been known for more than 70 years that liquid water accelerates the reaction between coal and atmospheric oxygen. In 1908, Dr. S. W. Parr (University of Illinois Bulletins 17 and 46) reported, "The presence of moisture increases the chemical reactivity of the coal-air system at any temperature." The quantitative effect for various coals has been extensively documented over the years. Ordinary combustion processes cannot take advantage of this phenomenon because wet coal must be dried before it will ignite.
Likewise, the catalytic effect of common alkalis such as soda ash (sodium carbonate) and limestone (calcium carbonate) on the reactivity of carbonaceous materials is well known and has been utilized in the gasification of coal and coke. Alkaline compounds are also used in commercial steam-hydrocarbon reforming catalysts to prevent carbon buildup by speeding up its oxidation to gaseous products. Conventional combustion processes do not employ alkaline catalysts because at the high temperatures they would volatilize and/or combine with ash ingredients to form troublesome slag or clinker.